Excitable membranes undergo rapid changes of ionic permeability following sudden changes of membrane voltage. These changes, which underlie the action potential, were described in detail by Hodgkin and Huxley (1952), who left no doubt that squid axon membrane can distinguish between sodium and potassium ions, and that permeability to both ions depends on membrane voltage. Little is known about the molecular structures and mechanisms involved in the permeability changes. The proposed work is directed toward achieving an understanding of membrane permeability and its dependence on membrane voltage in molecular terms. In particular we will seek to characterize and understand the recently described gating current (Armstrong and Bezanilla, 1974) which is a charge movement associated with the molecular rearrangements that attend the opening and closing of the sodium channels.